Solar System Dust 633
reducing its orbital energy. This effect is called Poynting–
Robertson effect. As a consequence of this drag force, the
particle is decelerated. This deceleration is largest at its per-
ihelion distance where both the light pressure and the veloc-
ity peak. Consequently, the eccentricity (aphelion distance)
is reduced, and the orbit is circularized. Subsequently, the
particle spirals toward the Sun, where it finally sublimates.
The lifetimeτPRof a particle on a circular orbit that spi-
rals slowly to the Sun is given byτPR= 7 × 105 ρsr^2 /Qpr,
whereτPRis in years,ris given in AU, and all other quan-
tities are in SI units. Even a centimeter-sized (s= 0 .01 m),
stony (ρ=3000 kg/m^3 , Qpr≈1) particle requires only
21 million years to spiral to the Sun if it is not destroyed by
an earlier collision. This example shows that all interplan-
etary dust had to be recently generated; no dust particles
remain from the times of the formation of the solar system.
The dust we find today had to be stored in bigger objects
(asteroids and comets), which have sufficient lifetimes.
The effect of solar wind impingement on particulates is
similar to radiation pressure and Poynting–Robertson ef-
fect. Although direct particle pressure can be neglected
with respect to radiation pressure, solar wind drag is about
30% of Poynting–Robertson drag.
Particle orbits that evolve under Poynting–Robertson
drag will eventually cross the orbits of the inner planets
and, thereby, will be affected by planetary gravitation. Dur-
ing the orbit evolution of particles, resonances with plane-
tary orbits may occur even if the orbit periods of the particle
and the planet are not the same but form a simple integer
ratio. This effect is largest for big particles, the orbits of
which evolve slower and which spend more time near the
resonance position. Density enhancements of interplane-
tary dust have been found (i.e., the Earth resonant ring was
identified inIRASdata and later confirmed by data from
the Cosmic Background Explorer satelliteCOBE).
Dust near other stars will also evolve under Poynting–
Robertson effect and form a dust disk around this star. Such
a disk has been found around many stars (e.g.,β-Pictoris).
There is an ongoing search in this disk for resonance en-
hancements that would indicate planets around this star
[SeeExtra-SolarPlanets.]
3.3 Collisions
Mutual high-speed (v>1 km/s) collisions among dust par-
ticles lead to grain destruction and generation of fragments.
By these effects, dust grains are modified or destroyed, and
many new fragment particles are generated in interplane-
tary space. From impact studies in stony material, we know
that, at a typical collision speed of 10 km/s, an impact crater
is formed on the surface of the target particle if it is more
than 50,000 times more massive than the projectile. This
mass ratio is strongly speed- and material-dependent. A
typical impact crater in brittle stony material (Fig. 5) con-
sists of a central hemispherical pit surrounded by a shallow
FIGURE 12 Schematics of meteoroid collisions in space. If the
projectile is very small compared to the target particle, only a
crater is formed in the bigger one. If the projectile exceeds a
certain size limit, the bigger particle is also shattered into many
fragments. The transition from one type to another is abrupt.spallation zone. The largest ejecta particle (from the spal-
lation zone) can be many times bigger than the projectile;
however, it is emitted at a very low speed on the order of
meters per second. The total mass ejected from an impact
crater at an impact speed of 10 km/s is about 500 times the
projectile mass.
However, if the target particle is smaller than the stated
limit, the target will be catastrophically destroyed. The ma-
terial of both colliding particles will be transformed into a
huge number of fragment particles (Fig. 12). Thus, catas-
trophic collisions are a very effective process for generating
small particles in interplanetary space. It has been found
that interplanetary particles bigger than about 0.1 mm in
diameter will be destroyed by a catastrophic collision rather
than transported to the Sun by Poynting–Robertson drag.3.4 Charging of Dust and Interaction with the
Interplanetary Magnetic Field
Any meteoroid in interplanetary space will be electrically
charged, and several competing charging processes deter-
mine the actual charge of a meteoroid (Fig. 13). Irradiation
by solar ultraviolet (UV) light frees photoelectrons, which
leave the grain. Electrons and ions are collected from the
ambient solar wind plasma. Energetic ions and electrons
then cause the emission of secondary electrons. Whether
electrons or ions can reach or leave the grain depends on
their energy and on the polarity and electrical potential of
the grain. Because of the predominance of the photoelec-
tric effect in interplanetary space, meteoroids are mostly
charged positively at a potential of a few volts. Only at times